1. Field of the Invention
The present invention relates generally to electric mass launchers, and more particularly to a railgun using an ablation resistant or composite insulator within the bore of a railgun electric mass launcher.
2. Description of the Related Art
Electric mass launchers have been investigated, in various forms, for several decades. One form of an electric launcher is called a railgun. A railgun is basically a linear motor for driving a projectile along a barrel. The barrel of the railgun is generally comprised of rails and an insulator material forming a bore, an armature located within the bore and an electrical power source which supplies the electrical current for driving the armature along the bore.
The railgun electric mass launcher 10, FIG. 1, uses a set of parallel conducting rails 12a and 12b separated by insulators (not shown), and a current shunt 14, called an armature which is free to slide between the rails 12a and 12b. Current 16 from an external source (not shown), such as a capacitor bank, battery bank, homopolar generator, or other high current power supply, is driven down one rail 12a, through the armature 14 and back along the other rail 12b. The insulators must be able to hold off the voltage between the rails 12a and 12b which can range from 10-500 v, depending upon the type of armature 14. The voltage across the rails 24a and 24b for a solid metal armature 14 is in the 10-50 v range. The magnetic field produced by the current 16 in the rails 12a and 12b applies a force on the current 16 driven through the movable armature 14. Such force is perpendicular to both the magnetic field direction and the direction of the current flow in the armature 14 and has a magnitude proportional to their product. In the railgun 10 this force is applied along the axis of the rails 12a and 12b, accelerating the armature 14 from the breech 18 to the muzzle end 19 of the railgun 10. The accelerated mass may be a solid mass such as an insulating jacket, called a sabot 21, encasing a solid projectile 22, as shown in FIG. 2. For low velocity applications (&lt;1.5 km/s) the sabot 21 is usually made of a sliding metal structure which makes good electrical contact with the conducting rails 24a and 24b. As the velocity increases into the 1-2 km/s range, the sliding metal-to-metal contact becomes resistive, due to localized arcs (not shown) which form between the contacts between the rails 24a and 24b, and the sliding metal sabot. Eventually the current 28 contact transitions into a single large area arc 26 which trails behind the solid armature. This arc 26, known as a plasma armature, is comprised of a high pressure (&gt;100 atmospheres), partially ionized gas distribution which fills the region between the conducting rails 24a and 24b. The current 28 is conducted between the rails 24a and 24b by this plasma distribution. Plasma armatures 26 are more resistive than solid metal armatures producing higher voltages across the arc 26. These voltages can be as high as 300-500 v depending on the current driven through the plasma. The magnetic field from the rails 24a and 24b causes a force on the current through the plasma armature 26 directed toward the muzzle 29. This force compresses the gas trapped between the arc 26 and the rear of the sabot 21 and the projectile 22 is accelerated down the bore 24 by the high pressure gas. Pressures driving the sabot 21 along the bore 24 can be -1,000 atmospheres allowing high velocities to be reached in relatively short barrels. Such railguns have been able to accelerate gram-sized masses to -8 km/s and 600 g masses to -4 km/s.
The materials that comprise the bore of the railgun are very important to its operation. In most cases it is the bore materials that limit the performance of the railgun. Local pressures in the armature can reach 1000 atmospheres over the plasma armature length which is typically several bore diameters long. Thus the bore walls, including the seal between the rails and the insulators, must be able to withstand the full armature gas pressure. Under these extreme pressures the rails and insulators tend to expand outward measurably, even with a high pre-load compression provided by the surrounding support structure. Thus the bore materials must be able to flex sufficiently to recover from the high gas pressure without losing their high pressure seal, cracking, or permanently deforming. In general metals are better at both resisting the pressure and at recovering from the localized flexing associated with the moving pressure distribution. Some insulators, such as ceramics, can withstand the pressure but tend to crack or craze under localized stress. Softer materials can compress and tend not to return to their original shape and size. The expansion can also affect the sliding high pressure seal between the bore and the projectile. If the rails and the insulators expand too much they can allow gas to escape forward past the projectile. This lowers the available pressure to accelerate a projectile. The escaping gas which blows by the projectile can also break down and lead to precursor arcs forming in front of the projectile. Such arcs can divert current from the armature and further decrease the acceleration. The smoothness of the bore wall surface is also important. The smoother the surface, the better the seal. Thus cracks or pits which are left on the bore wall must be smoothed or honed periodically. The wall material must resist damage while being able to be smoothed if necessary. Soft insulator materials or materials that crack under stress have to be replaced often which can severely restrict the utility of hypervelocity plasma armature railguns.
In addition to the high pressures the plasma armature also bathes the bore walls in an extremely high particle and radiation flux. The plasma armature has a small but finite resistivity. Current driven through the armature resistively deposits a significant amount of heat energy in the plasma. Ohmic heating rates can reach -300 megawatts during the typical 1-2 millisecond accelerating pulse. This energy heats the plasma in the armature which then reradiates the energy out of the armature plasma as black body electromagnetic radiation or hot particles. This radiated energy bathes the surrounding walls in the bore. Black body radiation from the hot gas distribution, which scales with the fourth power of the gas temperature, dominates the process. A equilibrium between the ohmic heating and the radiation is usually reached when the plasma temperature is 10,000.degree.-20,000.degree. K (1-2 eV). The armature is continually moving so a particular location on the rails or insulators is exposed to this radiated energy only when the armature passes. A typical location on the bore surface will see 100 s of kW/cm.sup.2 for times .about.100 microseconds. This amount of energy is sufficient to melt and ablate most materials from a surface. The threshold for ablation of the bore walls (heat flux necessary for ablation to occur for a given duration of exposure) depends on the wall material's heat capacity, heat conductivity, melting temperature, as well as the bore geometry. Likewise, the amount of material ablated from the surface once the ablation threshold has been reached depends on the amount of energy delivered to the surface as well as the heat of fusion (metal) or heat of formation (insulator) for the material. The heat of fusion or formation represents the energy required to release individual atoms of material from the surface. In general, metals have a relatively high ablation threshold (10-100 times larger) compared to the hydrocarbon-based insulators commonly used in railguns. Plastic insulators have ablation thresholds in the order of 10.sup.4 watts/cm.sup.2 while metals have ablation thresholds in the order of 10.sup.5 W/cm.sup.2. The amount of material ablated can be significant. The ablation products coming off the walls can enter the plasma arc and be accelerated along with the armature or can be left behind in the form of hot, partially ionized gas. Such hot gas trapped by the armature increases its length and mass and forces energy to be used accelerating the gas to the armature velocity. Hot gas left behind can lead to secondary arcs forming behind the plasma armature. Such secondary arcs divert current from he armature and slow the acceleration. Both cases, and other more subtle effects resulting from ablation, lead to a decrease in projectile acceleration. Thus control of the wall ablation is critical to optimizing the acceleration. The key to minimizing wall ablation is using ablation resistant materials for the rails and insulators.
A considerable effort has been expended trying to find the ideal material for rails and insulators. Metals are able to withstand high heat fluxes, but once the threshold for ablation is reached (100 s of KW/cm.sup.2) they begin to ablate large amounts of mass due to their low heat of fusion (energy needed to separate atoms from the metal lattice). Insulators in general have a much lower ablation threshold (10 s of KW/cm.sup.2) but a higher heat of formation (energy needed to break up the molecules into their constituents). This means that insulators, in general, start to ablate at a lower threshold level than metals but once the metal ablation threshold is reached, the metals will produce more ablated material than insulators. Most existing railguns use plastic (acrylic or epoxy resin) insulators which are particularly vulnerable to ablation. This results in rapid ablation of the surfaces and a flooding of the region in and around the arc with ablated hydrocarbons. Silicon carbide insulators have been tried but found to melt under the radiation flux and crack under the pressure. Diamond insulators are the best alternative, however, they cannot be fabricated in large volumes with existing technology. Machinable ceramics have relatively low ablation thresholds but tend to be brittle. Boron nitride has a high ablation threshold but tends to fill the bore with dust under high pressures. Epoxy materials like G-9 and G-10 (a form of fiberglass laminate bonded together with epoxy) are used routinely but produce copious amounts of ablated material. Such glass-epoxy materials also tend to delaminate under the high pressure and heat conditions in the barrel.
The ideal insulator would be a material that has a high ablation threshold as well as high heat of formation such as silicone carbide, alumina, or diamond. Unfortunately these materials are very expensive to make and tend to crack under extreme pressures.